As explained on the manual, in order to use GSX with any airplane that is not already internally supported by it, you first have to use the airplane configuration utility and set all door AND tell GSX how they are opened.
The default variables ( Exit 1 for the main passenger door, Exit 2 for the Cargo doors and Exit 3 for the Service doors ) are just a starting point that complies with the standard of the FSX default airplanes, but a 3rd party airplane can use different assignments or even custom XML variables, or might entirely lack animations and controls for a door.
Which is why the GSX editor allows you to specify either what standard FSX variable each door uses OR insert custom XML code in case of non-standard doors OR ignore the door status altogether (useful with static doors) so GSX won't check if the door is open or not, and will start services right away.